by While most of us take the ground beneath our feet for granted, within its complex layers, like the pages of a book, the history of the earth is written. Our history.
Research shows that there are little-known chapters in that history, deep in Earth’s past. In fact, Earth’s inner core appears to contain an even larger inner core.
“Traditionally, we have learned that the Earth has four main layers: the crust, the mantle, the outer core and the inner core,” geophysicist Joanne Stephenson of Australian National University explained in 2021.
Our knowledge of what lies beneath the Earth’s crust is largely derived from what volcanoes have revealed and what seismic waves have whispered.
From these indirect observations, scientists have calculated that the scorching hot inner core, with temperatures above 5,000 degrees Celsius (9,000 Fahrenheit), makes up only 1 percent of the Earth’s total volume.
But a few years ago, Stephenson and colleagues found evidence that Earth’s inner core may actually have two different layers.
“It’s very exciting – and could mean we have to rewrite the textbooks!” Stephenson explained at the time.
The team used a search algorithm to sift through thousands of models of the inner core and match them with observed data spanning many decades on how long it takes for seismic waves to travel through the Earth, collected by the International Seismological Center.
Differences in seismic wave paths through layers of the Earth. (Stephenson et al.Journal of Geophysical Research: Solid Earth, 2021)
So what’s down there? The team looked at some models of inner core anisotropy – how differences in material composition change the properties of seismic waves – and found that some were more likely than others.
While some models suggest that the material of the inner core channels seismic waves more quickly parallel to the equator, others indicate that the mix of materials allows for faster waves that are more parallel to the Earth’s rotation axis. Even then, there are debates about the exact degree of difference at certain angles.
The study here did not show much variation with depth in the inner core, but it did find that there was a change in the slow direction towards a 54 degree angle, with the faster direction of the waves being parallel to the axis.
“We have found evidence that may indicate a change in the structure of iron, perhaps pointing to two separate cooling events in Earth’s history,” Stephenson said.
“The details of this major event are still a bit of a mystery, but we have added a new piece of the puzzle when it comes to our knowledge of Earth’s inner core.”
These findings may explain why some experimental evidence is inconsistent with our current models of Earth’s structure.
The presence of an inner layer has previously been suspected, with evidence suggesting that iron crystals that make up the inner core have different structural alignments.
“We are limited by the distribution of global earthquakes and receivers, especially at polar antipodes,” the team wrote in their paper, explaining that the missing data reduces the certainty of their conclusions.
But their conclusions are consistent with other studies of inner core anisotropy.
Future research could fill some of these data gaps and allow scientists to confirm or contradict their findings, and hopefully translate more stories written in this early layer of Earth’s history.
This research has been published in the Journal of Geophysical Research.
An earlier version of this article appeared in March 2021.
